What happens if I remove the after cat 02 sensor and leave in the before one? The reason I ask is that the replacement is in-transit for a few days but I need to drive. Will the car just dump excess gas in the system?

 

you would still be able to drive it, but probably richer for the time being...

 

"The reason I ask is that the replacement is in-transit for a few days but I need to drive."
Huh?

 

X2.

You don't need 02 sensors for your jeep to start, run, drive, etc.

 

Running the engine without the rear O2 sensors in is the same thing as running your engine after changing the oil and then having two oil filter gaskets installed....BOOM!

j/k. You'll be fine, but I would just leave the defective sensors installed and drive the Jeep until the "in transit" replacement makes it to your house. You may run a little rich because the engine may stay in open loop, as opposed to closed loop where it reads information from the sensors.

 

The down stream O2 is there to monitor the catalytic converter efficiency, it does not make air/fuel adjustments. What is the code? If it is for the O2 heater circuit the sensor will still monitor the cat, the O2 heater is there for cold starts only.

 

Down stream O2 provides zero, zip, nada info to the PCM regarding a/f ratio......that's solely the job of the upstream O2. The down stream O2 monitors the cat, nothing more. Leaving the downstream O2 connected to the factory harness and tied up underneath should keep the CEL off 'til the new cat arrives.

Ooooops, Busted was posting while I was typing.....sorry.

 

The replacement o2 sensor is being shipped thus I plugged the port in the mean time. The vehicle still starts and drives but I'm getting a CEL and simply wanted to know what would happen. The CEL came on but then went off. It can't be a coincidence now that the o2 is missing.

 

Yep it will definitely throw the CEL with no rear o2. Mine hasn't had one since I bought it. Probably throwing po138 and po141. Just after you get the new sensor clear the codes and as long as you don't have a bad cat you should be good.

 

So how does your xj react without the sensor? Do you just drive with the CEL on?

 

i do this for years..just know that a code will come on..but i block it out,with black tape

 

Why take the time to plug it? Why not just leave the old one in until you get the new one? Yes, that is the reason the cel is on.

 

Why'd you remove it?

Anyway on a 1999 you'll get a CEL and that should be the most noticeable. The FSM is a little vague and some forum posters report improved mileage with a fresh D/S O2 but it's nothing at all to be concerned about.

 

Are you guys sure the rear 02 does not do anything for fuel mixture? I have a Mustang, and with almost all tuner upgrades they turn off the rear 02 sensors. It was always explained to me that they would read the adjusted fuel ratio and attempt to compensate, so that's why they turn them off. If all they did was monitor cat efficiency then why would they turn them off once the car has been tuned?

 

It really depends on the PCM. Your Mustang can't be used for reference.

There's a Chrysler article (well it might be Dodge branded) floating around that indicates that both O2S are used for setting the fuel trim. I forget what date is on it. The '97-'99 FSMs say "the downstream sensor is for monitoring catalyst efficiency", but also say "the oxygen sensorS are used for setting trim" (plural). Some people report a change when replacing the D/S, some don't. My 1999 does not report a B1S2 STFT to my scanner but I have not yet tried disconnecting the rear O2 and watching for a change in any other numbers (it may not be implemented in the PCM as a separate parameter).

2000+ Jeep 4.0L PCMs absolutely use all available sensors.